In the past, various clear plastic sheets (i.e., films) have been used to cover agricultural greenhouses and tunnels in an effort to enhance plant growth. Recently, in the cultivation of citrus fruits in particular, a technique called mulch cultivation has been adopted. Mulch cultivation is used to enhance the quality of citrus fruit by controlling the state of the cultivated soil (e.g., ground surface is covered to inhibit the growth of weeds). Mulch cultivation also minimizes the effect of the weather (e.g., heavy rains) at the time of cultivation and it protects the ground through which the roots of the trees pass. Typically, mulch cultivation is accomplished by covering the ground surrounding the citrus trees with a colorless transparent sheet or a silver or black opaque sheet made of polyethylene or a vinyl chloride-based plastic. In particular, silver sheets consisting of a polyethylene or vinyl chloride-based plastic on which aluminum foil has been vapor deposited are useful in the mulch cultivation of peaches.
A problem with such prior art plastic sheets is that they do not allow water to pass therethrough and they have relatively no air-permeability and thus do not allow carbon dioxide or moisture vapor to evaporate from the soil underneath the plastic sheet. In cases such as this, where evaporation of moisture vapor and carbon dioxide is obstructed, the tree is weakened and the quality of the fruit is diminished. Therefore, in order to work effectively, these plastic sheets must be laid down on the ground during rainy weather so as to cover the ground through which the tree roots pass, and either removed during fair weather or folded back so as to expose the ground and allow carbon dioxide and moisture vapor to escape. Repeating this type of work up until the time of harvest is a problem because predicting the weather is difficult and such labor is time consuming and inefficient.
Therefore, it is an object of the present invention is to provide a method of enhancing the quality of produce (e.g., fruits, vegetables and flowers) by using a particular kind of agricultural sheet material in place of the prior art materials currently in use. It is a further object of the invention to eliminate the above-described problems inherent in the prior art. In particular, the inventive method employs a spunbonded nonwoven film-fibril sheet material produced by a flash-spinning process which has excellent resistance to liquid water and yet excels in moisture permeability and air permeability. The spunbonded nonwoven film-fibril sheet material is able to improve the efficiency of the labor associated with mulch cultivation by not requiring sheet relocation during different weather conditions. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the drawings and the detailed description of the invention which hereinafter follows.